Hi Stu C!
If you are unable to send emails from a shared mailbox in Outlook using Office 365, and you have already tried adding the shared mailbox as an email account and making it default, there are a few additional steps you can try:
* Ensure that you have the necessary permissions: Before you can send emails from a shared mailbox, you must have the necessary permissions to do so. Ask the owner of the shared mailbox to grant you the appropriate permissions. You will need "Send As" or "Send on Behalf" permissions to send emails from the shared mailbox.
* Try using a different email client: If you are still unable to send emails from a shared mailbox in Outlook, try using a different email client that supports sending emails from shared mailboxes. You can try using the Outlook Web App (OWA) or a third-party email client.
* Try the Mail Merge in Word: Open Word and create your mail merge document. In the Mail Merge Recipients dialog box, click the Browse button and select the shared mailbox> Click OK.
In the Mail Merge Recipients dialog box, click the Edit button. In the Edit Recipients dialog box, select the checkbox next to the shared mailbox> Click OK.
Click Next: Write your letter. In the Write your letter section, type your letter. Click Next: Preview your letters. Preview your letters and make sure they look correct. Click Next: Complete merge. In the Complete merge section, select the checkbox next to Send E-mail Messages> Click OK.
Kindly let me know, if you require additional assistance, I will be glad to help further.
Best Regards, Shakiru